'error_handler' not set in Guardian pipeline

I’m trying to set up user authentication with Guardian 1.0 for my Phoenix 1.4 Absinthe api. Upon launching the server, I’m confronted with the following error from Graphiql:

# RuntimeError at POST /api

Exception:

    ** (RuntimeError) `error_handler` not set in Guardian pipeline
        (guardian) lib/guardian/plug/pipeline.ex:233: Guardian.Plug.Pipeline.raise_error/1

where my terminal output is:

[error] pid<0.496.0> running MyAppWeb.Endpoint (connection pid<0.492.0>, stream id 2) terminated
Server: localhost:4000 (http)                                                                                     
Request: POST /api                                                                                                
** (exit) an exception was raised:                                                                                
   ** (RuntimeError) error_handler not set in Guardian pipeline                                                
       (guardian) lib/guardian/plug/pipeline.ex:233: Guardian.Plug.Pipeline.raise_error/1                        
       (guardian) lib/guardian/plug/load_resource.ex:77: Guardian.Plug.LoadResource.return_error/3               
       (my_app) lib/my_app_web/router.ex:12: MyAppWeb.Router.graphql/2                                           
       (my_app) lib/my_app_web/router.ex:1: MyAppWeb.Router.pipe_through1/1                                  
       (phoenix) lib/phoenix/router.ex:270: Phoenix.Router.call/1                                            
       (my_app) lib/my_app_web/endpoint.ex:1: MyAppWeb.Endpoint.plug_builder_call/2                              
       (my_app) lib/plug/debugger.ex:122: MyAppWeb.Endpoint.“call (overridable 3)”/2                             
       (my_app) lib/my_app_web/endpoint.ex:1: MyAppWeb.Endpoint.call/2                                           
       (phoenix) lib/phoenix/endpoint/cowboy2_handler.ex:34: Phoenix.Endpoint.Cowboy2Handler.init/2              
       (cowboy) /home/spotless/Documents/AbsintheBook/OnlineTutorialBasics/tutorial/my_app/deps/cowboy/src/cowbo
y_handler.erl:41: :cowboy_handler.execute/2                                                                       
       (cowboy) /home/spotless/Documents/AbsintheBook/OnlineTutorialBasics/tutorial/my_app/deps/cowboy/src/cowbo
y_stream_h.erl:296: :cowboy_stream_h.execute/3                                                                    
       (cowboy) /home/spotless/Documents/AbsintheBook/OnlineTutorialBasics/tutorial/my_app/deps/cowboy/src/cowbo
y_stream_h.erl:274: :cowboy_stream_h.request_process/3                                                            
       (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3  

I’ve tried including the following line in my :graphql pipeline

plug Guardian.Plug.Pipeline, module: MyApp.Guardian, error_handler: Guardian.Plug.ErrorHandler

but I have not been successful. I’ve been following a tutorial which uses Guardian .14 and Phoenix 1.2, and I’ve been trying to transcribe different pieces of it with help from this community in previous posts. This is the last piece of the tutorial I’ll need help with to get it fully functional, and any help would be greatly appreciated I’ll leave relevant pieces of code below:

/lib/my_app_web/router.ex

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :graphql do
    # plug Guardian.Plug.Pipeline, module: MyApp.Guardian, error_handler: Guardian.Plug.ErrorHandler
    plug Guardian.Plug.VerifyHeader, realm: "Bearer"
    plug Guardian.Plug.LoadResource
    plug MyAppWeb.Context
  end

  scope "/", MyAppWeb do
    pipe_through :browser

    get "/", PageController, :index
  end

  scope "/api" do
    pipe_through :graphql

    forward "/", Absinthe.Plug,
      schema: MyAppWeb.Schema
  end

  forward "/graphiql", Absinthe.Plug.GraphiQL,
    schema: MyAppWeb.Schema

end

/lib/my_app/guardian.ex

defmodule MyApp.Guardian do
  use Guardian, otp_app: :my_app

  alias MyApp.Repo
  alias MyApp.User

  def subject_for_token(user=%User{}, _claims) do
    {:ok, "User:#{to_string(user.id)}"}
  end

  def resource_from_claims(claims) do
    "User: " <> user_id = claims["sub"]
    case Repo.get(User, user_id) do
      %User{} = user -> {:ok, user}
      _ -> {:error, :reason_for_error}
    end
  end
end

/lib/my_app_web/plugs/context.ex

defmodule MyAppWeb.Context do
  @behaviour Plug

  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _) do
    case Guardian.Plug.current_resource(conn) do
      nil -> conn
      user -> put_private(conn, :absinthe, %{context: %{current_user: user}})
    end
  end
end

Thanks in advance for any feedback!